Circuits and circuit designs are typically based on electrical and electronic components and properties and are useful for a variety of functions. An electrical circuit is an interconnection of electrical elements, such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources, and switches, and when it also contains active electronic components is known as an electronic circuit. Electronic circuits can usually be categorized as analog, digital or mixed-signal (a combination of analog and digital) electronic circuits. The basic units of analog circuits are passive (resistors, capacitors, inductors, and memristors) and active (independent power sources and dependent power sources). Components such as transistors may be represented by a model containing passive components and dependent sources. In digital electronic circuits, electric signals take on discrete values, which are not dependent upon time, to represent logical and numeric values. These values represent the information that is being processed. The transistor is one of the primary components used in discrete circuits, and combinations of these can be used to create logic gates. These logic gates may then be used in combination to create a desired output from an input.
In contrast, while some biological circuits have been developed, the utility of these circuits has been minimal, and it has been difficult to replicate the versatility and flexibility of standard electronic circuits. Such biological circuits have primarily utilized protein components to represent the state of memory of the cell. Such protein-based biological circuits are difficult to maintain and are unstable, as they require continuous protein expression. When compared to electronic circuits, such protein-based systems resemble DRAM (dynamic random access memory), which encodes volatile memory and requires power to maintain its state. Furthermore, such protein-based systems are not scaleable for use in biological circuits. Unlike electronic circuits, in which wires between physically separated components allow for spatial addressing, it is generally not possible to reuse the same biological component in protein-based systems. Hence, it becomes necessary to have different “parts” for every operation of the circuit, even for relatively elementary operations. Also, implementing all the parts necessary for such operations into a cell can place large energetic requirements on a cell. Finally, in such protein-based systems the various “states” of the circuit are encoded in transient chemical concentrations and cannot be maintained after cell death and cannot be easily transferred from one cell to another.
There is hence interest in the development and design of modular biological parts for the use in the development of biological circuitry. The development of biological systems in which the output depends both on the current inputs, as well as the input history, is a key requisite for complicated computation and information storage. Such biological circuitry can be used for a variety of purposes, including but not limited to, detection of cancers and toxins, counting of events, the design of biological computers, and the coding and reading of DNA fingerprints for engineered organisms.